Ink jet systems, and in particular multi-orifice, drop-on-demand ink jet systems, are well known in the art. A multi-orifice, drop-on-demand ink jet print head receives ink from an ink supply and ejects drops of ink through multiple orifices onto a print medium. Both thermal-type ink jet heads, which eject a drop by heating the ink to form a bubble, and impulse-type ink jet heads, which eject a drop by compressing a chamber, are common.
A thermal-type drop-on-demand ink jet print head is typically constructed by bonding together silicon wafers or hybrid thin film circuit substrates, the wafers or substrates having appropriate circuitry and chambers formed on their surfaces. An impulse-type drop-on-demand ink jet print head is typically constructed by bonding together multiple plates, the various chambers and channels being formed by appropriate holes in individual plates.
A typical impulse-type of multi-orifice drop-on-demand ink jet print head has a body that defines plural ink pressure chambers which are generally planar in the sense that they are much larger in cross-section than in depth. The ink pressure chambers each have an ink inlet and an ink outlet. The ink jet print head includes an array of proximately located nozzles and passages for coupling the ink pressure chambers to the nozzles. Each ink pressure chamber is Coupled by an associated passage to an associated nozzle. A driver mechanism is used with each pressure chamber to displace the ink in the ink chamber. The driver mechanism typically consists of a bending-mode pressure transducer, i.e., a piezo-ceramic material ("PZT") sandwiched between thin metal films and bonded to a thin diaphragm. When a voltage is applied to the PZT, it attempts to change its planar dimensions, but because the PZT is securely and rigidly attached to the diaphragm, bending occurs. This bending displaces ink in the ink chamber, causing the flow of ink both through an inlet from the ink supply to the ink chamber and through an outlet and passageway to a nozzle. Piston-like and shear-mode pressure transducers are also used as driving mechanisms for some ink jet printers.
The inlet of each pressure chamber is connected via a passage to a common ink manifold that supplies ink to the several pressure chambers. A restrictor orifice is sometimes positioned between the pressure chamber and the ink manifold to reduce acoustic crosstalk between pressure chambers. The use of such a restrictor orifice is described in U.S. Pat. No. 4,680,595 to Cruz-Uribe et al. for an Impulse Ink Jet Print Head and Method for Making Same.
For high resolution printing, it is desirable that the nozzles have very small orifices and be closely spaced. Close spacing requires correspondingly small internal channels. One method of achieving close spacing is described in U.S. Pat. No. 5,087,930 to Roy et al. for a Drop-on-Demand Ink Jet Print Head, which is hereby incorporated by reference in its entirety. Such small. orifices and internal channels in multiple orifice ink jet print heads are susceptible to clogging from particulate contamination that may be inadvertently introduced into the interior of the print head during assembly. Particulate contamination comes from various sources, such as the chromate layer on the ink reservoir, O-rings, bits of loose stainless steel from the various layers of the jet, and the assembly room environment.
U.S. Pat. No. 4,639,748 to Drake et al. illustrates an attempt to solve the particulate contamination problem. The patent describes a thermal ink jet print head constructed from two silicon wafers bonded together and having an integral ink filter. The integral filter, positioned between an internal ink reservoir chamber and capillary-filled ink supply channels, is formed by anisotropically and isotropically etching channels having cross-sectional areas smaller than the cross-sectional area of the nozzles into the silicon wall between the reservoir chamber and the supply manifold. Such fabrication methods are usable only for components fabricated from single crystal materials because other materials cannot be anisotropically etched to create the required structures. Also, such a filter cannot trap contaminants inadvertently introduced downstream from the manifold during assembly of the print head.
Another ink filter for a thermal ink jet print head is described in U.S. Pat. No. 4,864,329 to Kneezel et al. The print head described by Kneezel et al. comprises two silicon wafers, one of which has ink channels and an ink manifold having a fill hole. A wafer-sized, flat membrane filter is bonded to a silicon wafer surface over the fill holes to filter the ink before it enters the internal manifold of the print head. If the print head is constructed in the "roofshooter" configuration, i.e., the nozzles are located on the top surface of a silicon wafer, the membrane filter is positioned between the two silicon wafers and bonded to both. Such a filter, being an additional layer in the print head, increases the thickness and cost of the print head. Also, such a filter must be very flat to prevent ink from seeping around the filter. Additionally, this type of a filter is relatively far from the nozzle and, therefore, cannot trap many contaminants inadvertently introduced during assembly of the print head.
A Jolt Model printer by Dataproducts Corp. of Woodland Hills, Calif. uses a filter between the manifold and the inlet to the jets. However, particulate contamination trapped downstream of the manifold during assembly of the print head in this design can still clog the nozzles.
Filters are desirable to trap particulate contamination in ink jet print heads before such particulate contamination can clog the orifices or nozzles in the print head. However, existing filters are normally too remote from the nozzle to trap some of the particulate contamination introduced into print heads during assembly. Further, the use of filters carries the concomitant disadvantage of tending to restrict the flow of ink, thereby causing undesirable pressure drops.
Another difficulty encountered when using a particulate filter within a print head is the tendency of the small pores within the filter to trap bubbles. Purging is used to minimize this problem by applying a pressure drop across the filter to flush trapped bubbles out of the filter area during the purging operation. Practically it can be difficult to flush bubbles out of the filter because the ink typically lacks sufficient velocity and pressure at the filter location to transport the bubbles through the filter.